D5219: Use better description for size when size is not known

[Jan Grulich]

jgrulich added a reviewer: apol.

REPOSITORY
  R134 Discover Software Store

REVISION DETAIL
  https://phabricator.kde.org/D5219

To: jgrulich, apol
Cc: plasma-devel, progwolff, lesliezhai, ali-mohamed, jensreuterberg, abetts, 
sebas, apol